Stability, Formation and Dissociation Kinetics of the Pentacoordinate Ni 2+ and CO 2+ Complexes with 1,5‐Diazacyclooctane‐N,N′‐diacetic Acid

The stability constants of the Ni 2+ and Co 2+ complexes with 1,5‐diazacyclooctane‐N,N′‐diacetic acid (H 2 DACODA) have been determined potentiometrically in 0.5 M KNO 3 at 25°. Only M(DACODA) and M(DACODA)OH − were observed. In addition the formation and dissociation kinetics of the pentacoordinate complexes M(DACODA) has been studied in aqueous solution using a stopped‐flow technique. Formation follows the rate law v f = k f [M 2+ ] [HDACODA − ]/[H + ], which can be interpreted as a bimolecular process either between M 2+ and DACODA 2− (k   M,L ML ) or between MOH + and HDACODA − (k   MOH,HL ML ). The second order rate constants k   M,L MLare much higher than those expected from water exchange and can only be explained by a strong internal conjugate base effect. In the limiting case, however, this is equivalent to the second possible explanation, which assumes MOH + and HDACODA − as reactive species. The dissociation rate is given by v d = ( k ML + k   H ML[H + ]) · [M(DACODA)].